Accurate rotor position sensor and method using magnet and sensors mounted adjacent to the magnet and motor

ABSTRACT

A method for controlling a brushless electric motor including a rotor, a sense element with a plurality of magnetic pole pairs, and first and second sensors mounted less than or equal to 120 electrical degrees apart with sensing planes oriented perpendicular to an adjacent surface of the sense element and perpendicular to a movement direction of the sense element relative to the sensors, wherein one of a) the sense element and b) the first and second sensors is mounted in a fixed relationship with the rotor, the method including measuring magnetic flux of the plurality of magnetic pole pairs using the first and second sensors and outputting a corresponding measurement signal for each at least one first and second sensor, determining a rotational position of the rotor based on the measurement signals, and controlling the motor based on the determined rotor position.

This application is a continuation of copending U.S. application Ser.No. 09/118,980 filed on Jul. 20, 1998 now U.S. Pat. No. 6,522,130.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of electric motors, and inparticular to a sensor and method for accurately sensing a position of arotor in a brushless electric motor using a magnetic sense element andlinear output Hall effect sensors.

2. Description of Related Art

Electric motors that require controlled armature current waveforms (inorder to rotate smoothly, for example) also require accurate rotorposition sensing. Some motors use sensorless technologies, but thesetechnologies do not provide accurate rotor position sensing at very lowspeeds and are not smooth upon startup of the motor. Other motorsinherently cannot use sensorless technologies and must incorporate arotor position sensing mechanism. Currently, state of the art motors useeither an encoder or a resolver together with associated electroniccircuitry to determine rotor positions. Depending on the resolutionrequired, however, these solutions can become prohibitively expensivewithin applications that require low cost motors.

In particular, many electric motor applications require smooth rotationand/or accurate control. Brushless motors typically achieve this byusing 3-phase sine-wave commutation and accurate rotor positiondetectors, usually in the form of an encoder or a resolver. The accuraterotor position detector ensures that the sine wave remains synchronizedwith the rotor, thus avoiding commutation-induced torque ripple. Methodspresently used in the industry for accurately detecting rotor positionsuse encoders and resolvers and have been known and employed in motordrives for many years.

Encoders sense mechanical motion, and translate the sensed motion intoelectrical signals. Optical encoders are the most common type ofencoder. An optical encoder typically includes a housing to supportprecision bearings and electronics, a shaft with a disc that is calledan “optical disc” and has alternating clear and opaque segments, a lightemitting diode (LED), and a photo transistor receiver. A beam of lightproduced by the LED is aimed at the optical disc. When the optical discrotates, the light beam passes through the clear segments but is blockedby the opaque segments so that the optical disc effectively pulses thelight beam. The pulsed light beam is received by the photo transistorreceiver. The photo transistor receiver and the circuitry inside theencoder together provide signals to a motor controller outside theencoder and can also perform functions such as improving noise immunity.Encoders in their simplest form have one output to determine shaftrotational speed or to measure a number of shaft revolutions. Otherencoders have two outputs and can provide direction-of-rotationinformation as well as speed and number of revolutions. Still otherencoders provide an index pulse, once per revolution, which indicates anabsolute rotor position. The description thus far relates specificallyto incremental encoders, where upon startup, the position of the encoderis not known. A second type of encoder, called an absolute encoder, hasa unique value for each mechanical position throughout a rotation. Thisunit typically consists of the incremental encoder described above withthe addition of another signal channel that serves to generate absoluteposition information, typically of lesser accuracy. Within an absoluteencoder that is provided with an index pulse, the accuracy improves oncethe rotor traverses the index pulse. Incremental encoders can beacceptable within asynchronous motors, where speed feedback is mostimportant. Absolute encoders are desirable within synchronous motorapplications, where both position and speed feedback are important.

Another class of high resolution encoders is produced by severalcompanies, and is referred to as “sine/cosine encoders”. Sine/cosineencoders generate sine and cosine signals rather than pulse waveforms.When used with additional electronics, processor capability andsoftware, sine/cosine encoders indicate rotor position with fineresolution.

Encoders of all types are precision built, sensitive devices that mustbe mechanically, electrically and optically matched and calibrated.

Resolvers, on the other hand, typically provide one signal period perrevolution and are known to be highly tolerant of vibration and hightemperatures. A typical use of this technology would include a resolvergenerating two signals, both a sine-wave signal and a cosine-wavesignal, for each revolution. An advantage of using resolvers is thatthey provide absolute rotor position information, rather thanincremental information as is typical with most encoders. A primarydrawback, however, is that resolvers deliver increasingly poorperformance at low speeds. Because of this limitation, the speed controlrange possible with resolvers is much smaller than with encoders, on theorder of 200:1. Accordingly, use of resolvers is typically limited toapplications that do not require high quality motor control over a widespeed range. As with encoders, resolvers are precision built,commercially available sensing devices that can be fragile andexpensive.

Ring magnets and digital Hall effect sensors are often used as a rotorposition sensing mechanism within brushless direct current (DC) motorapplications where square-wave or six-step drive is used. This method ofsensing provides low resolution, typically six position steps perelectrical cycle when using three sensors. Six-step drive does notrequire high resolution rotor position sensing, however, so this isacceptable. At the same time, these drive methods do not result inripple-free torque from the motor either. This may be unacceptable in avariety of applications.

OBJECTS AND SUMMARY

Accordingly, a need exists for an accurate, low-cost device that sensesrotor position and detects rotational speed. According to an embodimentof the invention, this need is satisfied by providing an assembly thatincludes a magnetic sense element such as an inexpensive sense ringmagnet and two analog Hall effect sensors. In this embodiment the senseelement is fixed with respect to the motor rotor, and the sensors arefixed with respect to the motor stator.

The sense ring is magnetized in an alternating north-south fashion witha number of poles that corresponds to a number of motor field poles. TheHall effect sensors are placed so that they measure the magnetic fluxtangential to, and at some distance from, an outer circumference of thering.

Orienting the Hall effect sensors to measure magnetic flux tangential toan outer circumference of the ring and at some distance from the ringresults in a Hall effect sensor output voltage waveform that issubstantially triangular, with a highly linear portion centered at zeroflux, between the minimum and maximum peaks. This linear portion can bedecoded, e.g., using an analog-to-digital (A/D) converter and controlsoftware, into an accurate measure of rotor position. The cycle oroutput waveform repeats itself for every pole pair. For example, wherethere are two evenly spaced pole pairs, the output waveform of a Halleffect sensor will repeat twice for each mechanical revolution, i.e.,will have two complete electrical cycles. Accordingly, the inventivemethod can be used to decode rotor position within or relative to acomplete electrical cycle, but not necessarily within a completemechanical rotation that includes more than one electrical cycle, unlessan absolute position reference such as an index pulse is also provided.

The relationship between electrical and mechanical degrees is given as °E=° M·PP, where ° E represents electrical degrees, ° M representsmechanical degrees, and PP represents the number of magnetic pole pairsof the motor. By detecting absolute rotor position within a completeelectrical cycle, current can be controlled accurately at all times toresult in smooth rotation of the rotor.

According to another embodiment of the invention, the two Hall effectsensors can be placed further away from the sense ring, so that eachHall effect sensor outputs a substantially sinusoidal waveform. When thetwo Hall effect sensors are placed 90 electrical degrees apart, oneoutput becomes a sine wave and the other becomes a cosine wave.

Additional features and advantages of the invention will become apparentfrom the following description of the preferred embodiments, taken inconjunction with the accompanying drawings. The accompanying drawingsillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view along a radial direction, of magnetic fieldsgenerated by a multi-pole sense disc magnet that is magnetized in anaxial direction.

FIG. 2 is an end view along an axial direction of magnetic fieldsgenerated by a multi-pole sense ring magnet that is magnetized in aradial direction.

FIG. 3 is a graph of a magnetic field nearby and normal to an axial faceof the disc magnet of FIG. 1 or a radial face of the ring magnet of FIG.2.

FIG. 4 is a graph of a magnetic field parallel to an axial face of thedisc magnet of FIG. 1 or tangential to a radial face of the ring magnetof FIG. 2, that is measured at an air gap distance away from the disc orring.

FIGS. 5A and 5B show outputs of two Hall effect sensors located near asense disc or sense ring in a preferred embodiment of the invention, anda range over which the outputs are used to decode rotor position.

FIG. 6 is a graph of a magnetic field tangential to a face of the discmagnet of FIG. 1 or of the ring magnet of FIG. 2, at a distance greaterthan that shown in FIG. 4.

FIG. 7 is a side view of an electric motor with a sense ring inaccordance with an embodiment of the invention.

FIG. 8 is a side view of an electric motor with a sense disc inaccordance with an embodiment of the invention.

FIG. 9 is a side view of a electric motor with a sense ring inaccordance with an embodiment of the invention.

FIG. 10 is a front view of a conventional 3-wire Hall effect sensor.

FIG. 11 is a side view of an electric motor with two sense elements inaccordance with an embodiment of the invention.

FIG. 12 is a side view along a radial direction of a sense ring 100,that is similar to FIG. 1 except that the sensors 108, 109 are placed120 electrical degrees apart instead of 90 electrical degrees apart asin FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the invention, the magnetic sense element can beconfigured in a variety of shapes. For example, the magnetic senseelement can be a magnetized ring, i.e., sense ring, or a magnetizeddisc, i.e., sense disc, and can be axially magnetized or radiallymagnetized. In a first preferred embodiment of the invention, the ringor disc is magnetized with a number of magnetic poles that matches anumber of field poles of the motor. Setting the number of magnetic polesequal to the number of motor field poles generally simplifies theprocess of decoding Hall effect sensor output(s) to indicate rotorposition. The magnetic poles of the ring or disc alternate in polarityas shown in FIGS. 1 & 2. In particular, FIG. 1 shows a side view of anaxially magnetized disc 100, while FIG. 2 shows a radially magnetizedring 200. The waveforms shown in FIGS. 3-6 represent relationshipsbetween rotor position and measured magnetic flux, with rotor positionalong the horizontal axis and measured magnetic flux along the verticalaxis. FIGS. 7 and 8 show electric motors with a sense ring and a sensedisc, respectively, in accordance with embodiments of the invention.

FIG. 7 shows a motor 700 that has a stator 716, a rotor 724, and a shaft712 located inside a housing 710. The housing 710 supports the shaft 712via bearings 726. A sense ring 200 like that shown in FIG. 2 is affixedto the shaft 712 and rotates with the shaft 712 and rotor 724 about anaxis 714. A Hall effect sensor 722 is positioned near the sense ring 200by a support 720, so that the sensor 722 measures magnetic flux from thesense ring 200 that is tangential to an outer circumference 219 of thesense ring 200. See, e.g., FIG. 2, wherein the Hall effect sensors 108,109 are oriented to measure magnetic flux that is parallel or tangentialto the outer circumference 219, i.e., perpendicular to a radialdirection 202. The sensor 722 is fixed with respect to the stator 716.

The sense ring 200 includes magnets 204 arranged so that the ring 200 ismagnetized in directions radial from a rotational axis, e.g., in theradial direction 202. The ring 200 is provided with an inner ring orbacking material 206. The material 206 can be a softly magneticmaterial, for example a ferrous material like carbon steel, or can be anon-magnetic material, for example nylon.

FIG. 8 shows a motor 800 that is similar to that shown in FIG. 7, buthas a sense disc 100 like that shown in FIG. 1. A support 820 positionsthe Hall effect sensor 722 so that the sensor 722 measures magnetic fluxfrom the sense disc 100 that is perpendicular to the rotational axis 714and parallel with a face 119 of the sense disc 100. See, e.g., FIG. 1,wherein the Hall effect sensors 108, 109 are oriented to measuremagnetic flux that is parallel to a face 119 of the sense disc 100,i.e., perpendicular to an axial direction 102 about which the disc 100rotates. As in FIG. 7, the sensor 722 as shown in FIG. 8 is fixed withrespect to the stator 716.

The disc 100 includes magnets 104 arranged so that the disc 100 ismagnetized in the axial direction 102. The disc 100 is also providedwith a backing material 106 that is softly magnetic, such as a ferrousmaterial like carbon steel, or is non-magnetic.

Each magnetic pole of the sense ring or disc should be magnetizeduniformly in either the radial or axial direction so that nomagnetization “wave shaping” is necessary. The magnets can be made of aninexpensive material such as ferrite or bonded NdFeB.

The Hall effect sensors should be of the analog linear type, with asignal output that is linear over some range of magnetic flux (∓|B|).For example, commercial analog linear Hall effect sensors that aresuitable and currently available typically have linear magnetic fluxranges between about ∓500 gauss and about ∓1,500 gauss.

The sensors 108, 109 are aligned or oriented so that they measure themagnetic fields perpendicular to the magnetization directions 102, 202.In other words, the sensors are aligned to measure magnetic fluxtangential to the surface of the sense ring or disc.

As those of ordinary skill in the art will appreciate, a Hall Effectoccurs when, in the context of a three-dimensional coordinate systemwith three orthogonal axes x, y and z, an element carrying an electriccurrent in the x-axis direction is placed in a magnetic field whoseflux, or lines of force, are aligned parallel with the z-axis. Becausecharged particles passing through a magnetic field experience a Lorentzforce, the electrons traveling in the x-axis direction will be deflectedby a Lorentz force in the y-axis direction. This creates a chargeimbalance across the current-carrying element in the y-axis direction,and a corresponding voltage across the current-carrying element in they-axis direction.

A typical Hall effect sensor has a planar element oriented in the x-yplane, with a current flowing through the element in the x-axisdirection. When magnetic flux along the z-axis passes through the planarelement, a voltage will appear across the element in the y-axisdirection that is proportional to the magnetic flux passing through theplanar element. This voltage is the Hall effect sensor output.

In a situation where the Hall effect sensors 108, 109 are provided witha planar conducting element, the view in FIGS. 1 and 2 of the Halleffect sensors 108, 109 is along an edge of the planar conductingelement of each sensor. In other words, the plane of the element can bedefined by a rotational axis of the disc 100 or the ring 200 and aradial direction of the disc 100 or ring 200 that intersects and isperpendicular to the rotational axis. When the sensor 108 is located ata pole, as shown in FIGS. 1 and 2, no magnetic flux passes through thesensor 108 and the voltage across its planar element in the directionperpendicular to the current flow direction will be zero. As can be seenfrom FIGS. 1 and 2 with reference to the sensors 109, magnetic fluxpassing through the sensor 109 is at a maximum when the sensor 109 islocated equidistant between two adjacent magnetic poles, and a magnitudeof the voltage across its planar element in the direction perpendicularto the current flow direction will be at a maximum.

FIG. 10 shows a front view of a conventional Hall effect sensor 1000with three electrical leads 1002, 1004 and 1006. Typically, one of theleads is connected to ground, another is connected to a source voltage,and the third provides a sensor output voltage that indicates amagnitude and a direction or polarity of magnetic flux passing thethrough the planar face of the sensor 1000. Conventional 3-lead Halleffect sensors such as the sensor 1000 are typically configured so thatthe sensor output voltage on the third lead ranges from 0 volts to thesource voltage, where 0 volts represents a maximum magnetic flux with afirst polarity, and the source voltage represents a maximum magneticflux with the opposite polarity, and an output of half the sourcevoltage is a quiescent output representing zero magnetic flux passingthrough the sensor. The Hall effect sensors 108, 109 can be of thistype, or can be of any other suitable type of Hall effect sensor.

The Hall effect sensors 108, 109 can be located at a specified air gapdistance from the surface of the sense ring or disc so that a waveformoutput of each of the sensors 108, 109 will be quasi-triangular as shownin FIG. 4. The waveform output shown in FIG. 4 is a graph of sensoroutput voltage along the vertical axis, and position of the sensorrelative to the disc 100 or ring 200 along the horizontal axis. The airgap distance can be, for example, on the order of 100 mils (about 2.5millimeters) or less. An air gap distance at which the Hall effectsensor output will be quasi-triangular can vary depending on theparticular characteristics of the sense ring and Hall effect sensorused, and can be easily determined by experiment given specific devicecomponents and usage conditions. At a further distance, the waveformoutput becomes sinusoidal as shown in FIG. 6.

In contrast, traditional Hall effect sensor arrangements align the Halleffect sensors to measure magnetic flux in the direction ofmagnetization, normal to the sense ring, and output flux waveformssimilar to that shown in FIG. 3. For example, with respect to FIGS. 1and 2, a traditionally arranged Hall effect sensor would have its planarconducting element parallel to the surface 119 or tangential to thesurface 219, and would have a maximum output at a magnetic pole and aminimum output at a location equidistant between two adjacent magneticpoles.

As shown in FIG. 3, the waveform generated using traditional Hall effectsensor arrangements is linear, i.e., has a constant slope, for only asmall portion of the waveform period near the zero flux crossings. Inthe remainder of the waveform period, the waveform shape curves and isthen relatively flat (i.e., has a slope with a small value) for a largeportion of the waveform period near the waveform extremes. This waveformshape is will not produce accurate rotor position detection for severalreasons.

First, the waveform shape shown in FIG. 3 is undesirable because it islinear for only a small portion of the waveform period. In contrast, anideal waveform shape would have linear slope for at least one half ofthe waveform period. A linear shape is desirable because converting ordecoding a voltage level into a rotor position can be done simply andconsistently when the voltage level changes linearly with rotorposition.

Second, the waveform shown in FIG. 3 is relatively flat for a largeportion of the waveform period, near the waveform extremes; this isundesirable because the ratio of voltage change to rotor position changebecomes smaller as the waveform slope flattens. This small ratiorequires greater measurement sensitivity and increases the system'svulnerability to noise.

Third, for the portions of waveform period where the waveform shape iscurved, the shape of the curve must be known and additional computationmust be performed using the shape of the curve to accurately determinethe rotor position.

FIG. 4 shows a Hall effect sensor output waveform that is much moredesirable than the waveform shown in FIG. 3. As shown in FIG. 4, thewaveform shape surrounding the zero flux crossing is linear, while thewaveform shape at the peaks is slightly rounded. As shown in FIG. 4,well over half of the waveform shape is linear. A preferred embodimentof the invention avoids using the rounded waveform shape at the peaks todetermine rotor position by using two linear Hall effect sensors spaced90° electrical apart. Rotor position information is supplied by bothsensors in an alternating fashion, as shown for example in FIGS. 5A and5B, so that only linear portions of the sensor output waveforms areused.

Since the sensor output waveforms are non-linear only near their peaks,non-linear portions of the waveforms can be identified by comparing asensor's output to a threshold value which is less than or equal to amagnitude below which the waveform is linear, and above which thewaveform is non-linear.

As can be seen in FIGS. 4 and 5A, the waveform of each sensor isgenerally linear within about 60° on either side of a zero crossing ofthat waveform. Since the two sensors (and therefore their respectivezero crossings) are spaced 90 electrical degrees apart, as for examplethe Hall effect sensors 108, 109 shown in FIGS. 1 & 2, this means thatboth sensors 108, 109 will simultaneously have a linear output in aregion midway between two adjacent zero crossings, where one of theadjacent zero crossings is a zero crossing for the sensor 108 and theother adjacent zero crossing is a zero crossing for the sensor 109,e.g., two adjacent zero crossings of the waveforms 550 and 560 of FIG.5A. Since the distance between the sensors 108, 109 (and thus thewaveforms 550 and 560) is 90 electrical degrees, and since each of thewaveforms 550, 560 is substantially linear within 60 electrical degreeson either side of its zero crossing, the width of each overlap regionwhere both waveforms 550, 560 are simultaneously linear is 30 electricaldegrees. Thus, at any position at least one of the sensors 108, 109 willhave a linear output of voltage with respect to a position of the senseelement relative to the sensor, and at some positions (between 30 and 60electrical degrees distant from each waveform zero) both sensors 108,109 will have a linear output.

If sensors are used wherein each sensor's output waveform has a linearregion covering less than 60 electrical degrees on either side of a zerocrossing for that sensor, then the overlap region where both sensorshave a linear output will be correspondingly smaller. Where each of thetwo sensor waveforms is linear within 45° of a zero crossing of thatwaveform, but is non-linear further than 45° from the zero crossing(until it is within 45° of the next zero crossing for that waveform),then since the two sensors 108, 109 are spaced 90 electrical degreesapart the linear regions of the two waveforms will not overlap althoughat any location one of the waveforms will be linear.

A simple solution is to choose the threshold value to equal a waveformvalue that occurs at plus or minus 45 electrical degrees from a zerocrossing of the waveform. With this threshold value, as shown in FIGS.5A and 5B, at any point in time one and only one of the two sensors 108,109 will have an output that is below the threshold value. Thus, a motorcontroller can use this threshold value to easily determine which sensorto heed for rotor position information.

In particular, as shown in FIG. 5A one of the two sensors 108, 109outputs the waveform 550, and the other sensor outputs the waveform 560.The vertical axis represents voltage, and the horizontal axis representsposition, for example, positions of each sensor with respect to thesense element. A reference line 574 is located 45 electrical degreesfrom an origin 576, which is also a “zero point crossing” for thewaveform 550. The waveforms 550, 560 are linear between the thresholds570 and 572. Linear segments of the waveforms 550, 560 that are locatedbetween the thresholds are labeled 550A-F and 560A-F. Reference lines578 and 580 are located 45 electrical degrees from zero point crossingsof the waveforms 550 and 560. As can be seen in FIG. 5A, each positionalong the horizontal axis corresponds to a point on only one of segments550-A-F and 560A-F. FIG. 5B is similar to FIG. 5A, but omits portions ofthe waveforms 550 and 560 that fall outside the thresholds 570 and 572so that the linear portions of the waveforms that are used by the motorcontroller, i.e., the segments 550A-F and 560A-F, can be more easilyseen.

Each of the Hall effect sensors 108, 109 will output the quiescent valueV_(quiescent) when there is no magnetic flux passing through the Halleffect sensor. For example, the Hall effect sensor 108 has no magneticflux passing through it, and thus will output the quiescent value. Whenone of the Hall effect sensors 108, 109 has a maximum amount of magneticflux passing through it, as for example the Hall effect sensor 109 shownin FIG. 1, it will output one of the minimum value V_(min) or themaximum value of V_(max). Whether it outputs the maximum or the minimumvalue depends on the direction in which the magnetic flux passes throughthe Hall effect sensor 109. Since the magnets 104 alternate polarity andthus magnetic flux direction, outputs of the Hall effect sensors 108,109 will also alternate between the minimum and maximum values as, forexample, the Hall effect sensors 108, 109 move through the magneticfields in the direction 121 shown in FIG. 1.

When the Hall effect sensors 108, 109 are of the conventional 3-leadtype described above with respect to FIG. 10, the maximum value V_(max)of the output waveforms 550, 560 is an input voltage V_(in) provided tothe corresponding Hall effect sensors, the quiescent value V_(quiescent)or “zero” of the output waveforms 550, 560 is half the input voltage orV_(in)/2, and the minimum value V_(min) is zero volts.

An absolute rotor position within an electrical cycle can be determinedusing the two sensors. As shown for example in FIG. 5A with respect tothe waveforms 550, 560, for each waveform, all values of the waveform(except for the minimum and maximum values) occur twice in an electricalcycle. For example, for a given value of the waveform 550 that isbetween the quiescent voltage V_(quiescent) and the threshold 570, theposition can lie on either the segment 550A or on the segment 550B. Whena value of the waveforms 550, 560 that is between the thresholds 570,572 is being used to indicate rotor position, the value of the other oneof the waveforms 550, 560 can be used to determine which one of the twosegments of the first waveform should be used. For example, where avalue of the waveform 560 lies above the quiescent voltage but below thethreshold 570, and the corresponding value of the waveform 550 isgreater than the quiescent voltage, the rotor position corresponds tothe linear segment 560A rather than the linear segment 560B. Thus, thewaveforms 550, 560 together indicate an absolute position of the rotorwithin an electrical cycle.

In the embodiment described above two sensors are located 90 electricaldegrees apart, and the thresholds 570, 572 are chosen so that eachlocation along the horizontal axis corresponds to one point on a linearwaveform segment.

However, other configurations can be used. For example, a distancebetween sensors can be adjusted to an appropriate value that isdifferent from 90 electrical degrees, more than two sensors can be used,and thresholds can be set differently. Reasons for using a differentconfiguration can include, for example, using sensor output waveformsthat have different linear regions. The linear regions can varydepending on characteristics of the sensors, air gap distances betweenthe sensors and the sense element, and other factors. Configurationshaving a) points on linear waveform segments for only some sensor-senseelement positions, b) multiple points on linear waveform segments foronly some sensor-sense element positions, or c) multiple points for eachposition, can also be variously useful or desirable depending onparticular applications of the invention.

The preferred embodiment uses a microprocessor based controller to takethe analog signals provided by the linear Hall effect sensors andconvert them to digital signals using an A/D converter. Thisconfiguration can provide high resolution rotor position sensing. Forexample, assume that a motor contains eight pole pairs and that eachdiagonal line in FIG. 5 equates to 256 rotor position steps. Since thereare eight electrical cycles per rotation and four lines per electricalcycle, the total resolution per rotation equals 8192 (256×8×4). This isexcellent resolution within a motor-driven system. Another significantadvantage is that this system can be incorporated into a motor forlittle cost.

In another embodiment of the invention, the air gap between the sensering and the sensor is increased so that the sensor output describes asubstantially sinusoidal waveform, as shown in FIG. 6. In particular,FIG. 6 shows output waveforms of two sensors located 90 electricaldegrees apart, where the waveform 630 corresponds to a first sensor, andthe waveform 632 corresponds to a second sensor. Position informationcan be decoded or extracted from the substantially sinusoidal waveforms630, 632 shown in FIG. 6 by applying principles well known in the art,similar to the “sine-cosine” method used within some conventionalresolvers. As in the triangular waveform method described above withrespect to FIGS. 4 & 6, portions of each waveform nearest the zero-fluxcrossings of the waveform can be the primary signals used to decode therotor position.

In another embodiment of the invention, a single analog switch, i.e.,Hall effect sensor, is used with one sense ring. Linear portions of thesensor output can be used as described above to determine rotorposition. Nonlinear portions of the sensor output near the waveformpeaks can either be ignored, or can be used to estimate rotor position.Rotor speed and acceleration information can also be used to estimatethe rotor position during a time period in which the sensor output isnonlinear.

In another embodiment of the invention, two sense rings and at least twoand preferably three Hall effect sensors are used. One ring is used tosense high resolution increments, and the other ring is used to senseabsolute position. Where three sensors are available, two of the sensorscan be used with the high resolution sense ring to provide an indicationof absolute rotor position within an electrical cycle, and a thirdsensor can be used with the absolute position sense ring to provideinformation that can be used to indicate within which electrical cycleof the mechanical revolution the rotor is located. The high resolutionsense ring preferably has a number of magnetic poles that equals anumber of motor field poles, or equals an integer multiple of the numberof motor field poles. The absolute position sense ring preferably hastwo poles (magnetic north and magnetic south). The poles can have thesame size, for example each pole occupies half or 180° of the sensering. Alternatively, one pole can occupy a large portion of the absoluteposition sense ring, and the other pole can occupy a remaining, smallerportion of the ring.

For example, FIG. 11 shows a motor 1100 that is similar to those shownin FIGS. 7 and 8, but has two sense elements. The sense elements can besense rings. For example, FIG. 11 shows a sense ring 200 and a sensering 1130 that is similar to the sense ring 200, and includes magnets1104, an inner ring or backing material 1106, and has a circumferentialface 1119. A Hall Effect sensor 1122 is positioned near the sense ring200 by a support 1120, so that the sensor 1122 measures magnetic fluxfrom the sense ring 200 that is tangential to an outer circumference 219of the sense ring 200, and a Hall Effect sensor 1123 is likewisepositioned near the sense ring 1130 by a support 1121. The sensors 1122and 1123 are fixed with respect to the stator 716. One of the senseelements can be used as an absolute position sense element, and theother sense element can be used as a high resolution sense element.

As a further alternative, at least one sensor can be used with the highresolution sense ring, and two sensors mounted 90° apart can be usedwith an absolute position sense ring having two poles so that anabsolute mechanical position of the rotor can be determined at any pointin time.

As yet a further alternative, a different method that does not employ aHall effect sensor oriented to measure magnetic flux tangential to thesurface of the sense ring or disc, can be used to supply an absoluteposition signal once per mechanical revolution of the rotor.

In another embodiment, an analog signal from a Hall effect sensor isused directly to sense rotor position instead of a digital signal thatis based on the analog signal and obtained by supplying the analogsignal to an A/D converter.

According to an embodiment of the invention, the Hall effect sensor(s)can be fixed with respect to the motor rotor, and the sensing elementcan be fixed with respect to the motor stator.

According to an embodiment of the invention, the number of magneticpoles of the sense element is different than the number of motor fieldpoles. For example, A greater number of magnetic poles of the senseelement than motor field poles can be provided. Generally, where thesense element poles are used to determine an incremental position of themotor rotor with respect to the motor stator, increasing the number ofsense element poles increases the accuracy and resolution of thedetermined incremental position.

The sense ring or disc can, for example, be made out of ferrite, bondedNdFeB, sintered NdFeB, or SmCo. Other sensors besides Hall effectsensors that also generate a substantially triangular or sinusoidaloutput waveform that is a graph of position vs. sensor output, can beused instead of, or in addition to, Hall effect sensors.

FIG. 9 shows another embodiment wherein a motor 900 is similar to themotor 700 shown in FIG. 7, but differs in that the sense ring 902 isformed in a cup shape, so that the magnets 904 are disposed on an innerdiameter of a rim of the cup 906, which is formed of soft magnetic ornon-magnetic material. A support 920 holds a Hall effect sensor 922 nearan inner diameter of the ring formed by the magnets 904 along the rim ofthe cup 906.

Although the invention has been described in detail with reference onlyto presently preferred embodiments, those skilled in the art willappreciate that various modifications can be made without departing fromthe invention. Accordingly, the invention is defined only by thefollowing claims which are intended to embrace all equivalents thereto.

What is claimed is:
 1. A method for controlling a brushless electricmotor including a rotor, a sense element with a plurality of magneticpole pairs, and first and second sensors mounted less than or equal to120 electrical degrees apart with sensing planes oriented perpendicularto an adjacent surface of the sense element and perpendicular to amovement direction of the sense element relative to the sensors, whereinone of a) the sense element and b) the first and second sensors ismounted in a fixed relationship with the rotor, the method comprising:measuring magnetic flux of the plurality of magnetic pole pairs usingthe first and second sensors and outputting a corresponding measurementsignal for each of the first and second sensors; determining arotational position of the rotor based on the measurement signals; andcontrolling the motor based on the determined rotor position.
 2. Themethod of claim 1, wherein the first and second sensors are Hall effectsensors.
 3. The method of claim 1, wherein the determining comprisesdetermining an absolute rotational position of the rotor within anelectrical cycle based on the measurement signals.
 4. The method ofclaim 1, wherein the first and second sensors are mounted at an intervaldifferent from 90 electrical degrees.
 5. The method of claim 1, whereinone of the measurement signals of the first and second sensors indicatesa region within an electrical cycle, and the other of the measurementsignals of the first and second sensors indicates a location within theregion.
 6. The method of claim 1, wherein the first and second sensorsare mounted less than 120 electrical degrees apart.
 7. The method ofclaim 1, wherein the first and second sensors mounted with sensingplanes oriented perpendicular to a movement direction of the senseelement relative to the sensors.
 8. A method for controlling a brushlesselectric motor including a rotor, a sense element with a plurality ofmagnetic pole pairs, and first and second sensors mounted with theirsensing planes perpendicular to a surface of the sense element tomeasure magnetic flux from the magnetic poles, wherein one of a) thesense element and b) the first and second sensors is mounted in a fixedrelationship with the rotor, and wherein the first and second sensorsare spaced so that at each rotational position of the rotor, an outputof at least one of the first and second sensors is linear, the methodcomprising: measuring magnetic flux of the plurality of magnetic polepairs via the first and second sensors and outputting a correspondingmeasurement signal for each of the first and second sensors; determiningan absolute rotational position of the rotor within an electrical cyclebased on the measurement signals; and controlling the motor based on thedetermined rotor position.
 9. The method of claim 8, wherein the firstand second sensors are mounted with their sensing planes orientedperpendicular to a rotation direction of the sense element relative tothe sensors.
 10. The method of claim 9, wherein the first and secondsensors are Hall effect sensors.
 11. The method of claim 8, wherein oneof the measurement signals of the first and second sensors indicates aregion within an electrical cycle, and the other of the measurementsignals of the first and second sensors indicates a location within theregion.